FIBRILLATED PTFE SURFACE

Abstract: A porous polytetrafluorethylene (PTFE) structure suitable for use as an oil transfer component for coating or wiping a roll in a fuser system of a copying machine, has a fibrillated surface produced by casting a PTFE suspension onto a mesh and drying such that the PTFE engulfs at least some of the wires of the mesh. Removal of the PTFE sheet from the mesh results in a fibrillated irregular surface (72, 74). The PTFE structure is formed from particles of granular-type polytetrafluoroethylene fused together to form a porous integral network of interconnected particles.

Description: FIBRILLATED PTFE SURFACE FIELD OF THE INVENTION The present invention relates to a porous polytetrafluoroethylene structure having a fibrillated surface. It may particularly though not exclusively be used as an oil transfer component for applying release oil to a fuser roll in a copying machine, and/or wiping excess toner therefrom or from another roll in contact therewith. The term "copying machine" as used herein relates to machines which employ heated fuser rolls, for example plain paper copying machines and laser printers. PRIOR ART In a plain-paper copying machine, toner images applied to the surface of paper or other recording medium are fixated by application of heat and pressure. In certain plain paper copying machines fixation is accomplished by passing the image-bearing recording medium between a hot thermal fixation roll and a pressure roll. When this type of thermal fixation device is used the toner material is directly contacted by a roll surface, and a portion of the toner usually becomes adhered to the roll surface. On subsequent rotation of the roll, the adhered toner material may be redeposited on the recording medium resulting in undesirable offset images, stains, or smears; or in severe cases the recording medium may stick to the adhered toner material on the roll and become wrapped around the roll. To counter these problems, materials having good release properties such as silicone rubber or polytetrafluoroethylene are often used for the roll surfaces. Although improving performance of the thermal fixation devices, use of silicone rubber or polytetrafluoroethylene roll surfaces alone does not eliminate the problem. Another approach is to include release agents with the toner materials, which prevent the toner materials from adhering to the roll surface. These oil-less toners improve the performance of the thermal-fixation devices but again, particularly in the case of high-speed type copying machines, do not completely eliminate the problems associated with toner pick-up and transfer. Toner pick-up by the rolls can be controlled by coating the surface of at least one of the rolls with a liquid release agent, such as a silicone oil. It is important that the release liquid be applied uniformly and in precise quantities to the surface of the roll. Too little liquid or non-uniform surface coverage, will not prevent the toner from being picked up from the paper and deposited on the roll. On the other hand, excessive quantities of the release liquid may cause silicone rubber roll surfaces to swell and wrinkle, thus producing copies of unacceptable quality. Devices to uniformly meter and coat a release liquid onto copy machine roll surfaces are described in Japanese laid-open patent No. 62-178992. These devices consist of an oil permeation control layer adhered to a thick porous material which serves as a wick or reservoir for supplying oil to the permeation control layer. The permeation control layer is typically a porous polytetrafluoroethylene film which has been impregnated with a mixture of silicone oil and silicone rubber followed by a heat treatment to cross-link the silicone rubber. The thick porous material to which the permeation control layer is adhered is typically a porous thermosetting resin foam or a felt of Nomex (Trademark) fibres, glass fibres, carbon fibres, polytetrafluoroethylene fibres, or any other high temperature fibre. Fibres sold under the NOMEX trademark are aramid fibres, a type of polyamide. Conventional plain paper copiers which use silicone oil as the liquid release agent generally comprise a means for coating the silicone oil onto the fuser roll and also a means for wiping the roll to remove undesirable adhered toner, paper dust and other undesirable material. In many cases, the coating and wiping functions are performed by a single device. In one particular type of machine, there is provided a reservoir wick which is generally formed of a Nomex felt, with a cover wick placed over the top of it for metering the flow of oil from the reservoir wick to the fuser roll and also for providing improved wear and distribution characteristics. The cover wick is typically a Nomex fabric. It is found, however, that the cover wick has only a limited lifetime, typically lasting for around 60,000 copies before requiring replacement. It would be desirable to have a cover wick which lasts as long as the life of the fuser rollers. In another commonly used type of copying machine, the oil coating and wiping function is provided by a length of felt, typically formed of Nomex or polytetrafluoroethylene fibres. The roll is typically 435 millimetres wide by 1.4 metres long. Whilst the machine is copying, the roll, which is laden with silicone oil, travels past the fuser roll at a speed of about 2cm per hour. This oiling roll applies silicone oil. Also excess toner cleaned from the fuser roll is picked up and carried away on the oiling roll. Typically, the felt of the oiling roll has a weight of 700 grammes per square metre and a thickness of 1.3 milimetres. Due to the physical restraints of the copying machine, this limits the length of material on the oiling roll to about 1.4 metres. It is generally speaking not possible to make satisfactory felts which are lighter than 700 grammes per square metre for this purpose. In practise, it is therefore necessary to replace the oiling roll about every 10 days of operation. British published patent application 2242431 (9106768.6) discloses a porous polytetrafluoroethylene structure for use as a filter in industrial filtration. The porous polytetrafluoroethylene material is produced by fusing particles of polytetrafluoroethylene such as to form a porous integral network of interconnected particles. The disclosure of this patent specification is incorporated herein. The specification is particularly concerned with the filtration of aqueous slurries. Our published patent application W093/08512 discloses the use of such porous PTFE material as an oil transfer component in a copying machine and particularly as an oil permeation control layer to control the amount of release oil applied to the fuser roll, and also to wipe excess toner therefrom. SUMMARY OF THE INVENTION It has now been surprisingly found that the properties of the porous PTFE structure can be improved by providing a fibrillated surface thereon. Thus, one aspect of the invention provides a porous polytetrafluoroethylene (PTFE) structure formed from particles of granular-type polytetrafluoroethylene fused together such as to form a porous integral network of interconnected particles, the structure having a fibrillated surface. Generally speaking, the present invention envisages the use of a porous polytetrafluoroethylene material such as disclosed in GB 2242431 for use as an oil transfer component for coating or wiping a fuser roll in a copying machine. The material is able to withstand the high temperatures (around 200°C) encountered and has excellent mechanical properties and durability in this application. A further aspect of the invention provides an oil transfer component for coating or wiping a roll in a fuser system of a copying machine which comprises a porous polytetrafluoroethylene structure formed from particles of granular-type polytetrafluoroethylene fused together such as to form a porous integral network of interconnected particles, the structure having a surface for contacting said fuser system roll in use, the surface being fibrillated. In order to act as an oil transfer component, the porous polytetrafluoroethylene (PTFE) structure will generally include a support means for supporting the structure within the copying machine. In the case of a so-called cover wick, the support means may be in the form of a frame or pair of parallel rods extending along either side of the cover wick strip. Alternatively, the porous polytetrafluoroethylene structure may be laminated to a reservoir material acting as a support structure. The porous polytetrafluoroethylene structure may also be provided in the form of a continuous web, or may be applied onto a roller formed of an oil reservoir material (such as a foam as disclosed in Japanese 62-178992 or a porous ceramic) which is optionally covered by a felt cover. Generally, the porous polytetrafluoroethylene structure is in the form of a sheet material, typically of a thickness in the range 50 microns to 1500 microns, especially 100 to 1,000 microns. The porous polytetrafluoroethylene structure is hydrophobic but has a high affinity for liquid release agents such as silicone oil. The oil transfer component will generally be supplied pre-loaded with release oil. In an oil-filled type the component is discarded when this oil is substantially used up. In an oil-fed type further oil is supplied thereto by means of an oil delivery mechanism. Typically, the oil will constitute 10 to 50% by weight of the PTFE structure of the oil transfer component, particularly 20 to 40% by weight. In order to provide such oil retention capacities, the porous polytetrafluoroethylene structure usually has a specific gravity of 0.5 to 1.8, for example 0.6 to 1.5, typically 0.7 to 1.2 measured as described herein. In comparison, pure non-porous PTFE typically has a specific gravity of 2.16. Preferably, the porous polytetrafluoroethylene structure does not include any filler materials, since these are generally of mineral origin and tend to be of an abrasive nature which would damage the fuser rolls. According to the invention, the structure has a fibrillated surface. This means that elongate fibrils of PTFE extend from the surface of the structure. The fibrils are generally up to 2mm in length, particularly up to 1mm long, and especially up to 0.5mm long. If the fibrils are too long, there is a greater risk of the fibril becoming detached from the surface of the structure in use. On the other hand if they are too short, then their coating and wiping ability may be reduced. Fibrils of length 0.1 to 0.5mm long are preferred. The length of each fibril is generally 1 to 10 times its transverse dimension. Generally, the fibrils are produced by tearing from the surface of the PTFE structure. The fibrils have a form which is unitary with the structure of the PTFE, and are therefore firmly anchored into the PTFE structure as a whole. However, in a preferred embodiment, the porous PTFE structure is first bonded onto a substrate having an irregular surface which mechanically binds the PTFE structure to such an extent that when the PTFE structure is pulled way from the substrate the PTFE surface is torn and fibrils pulled out of the surface. The surface of the substrate may be provided with a pattern of surface features such as ridges, valleys, indentations, protrusions etc. which may be in a regular or non-regular pattern. Generally, the surface features are steep sided or have undercut portions so as to aid mechanical locking of the PTFE to the substrate. A particularly useful substrate has been found to be a wire mesh, preferably of stainless steel. Generally the mesh has 15 to 75 wires per inch in one direction (of a thickness 1.5 to 0.3mm each wire) ; and 75 to 400 wires per inch in a direction at right angles (of a thickness 0.3 to 0.07mm) . The mesh apertures may be in the range 40 to 150 microns across. A fabric could also be used in place of the wire mesh, provided the fabric had adequate mechanical strength to be removed from the PTFE structure. For best fibrillation, at least some of the wires of the mesh should be engulfed by the PTFE so that the wires are torn out of the PTFE on removal. Where the mesh has thicker and thinner wires, preferably only the thinner wires are engulfed by PTFE. The PTFE may be deformed (e.g. between embossed rollers) into intimate contact with the substrate, but preferably the PTFE structure is formed of granular PTFE particles in a liquid vehicle sprayed or otherwise laid down onto the substrate prior to fusion of the particles. When the PTFE is removed from the substrate the surface features of the substrate may be entirely converted into fibrils on the PTFE surface. However, in a preferred embodiment the PTFE surface has an overall surface texture (as disclosed in our W093/08512) in addition to the fibrils. In the case of a wire mesh substrate this would manifest itself as an overall pattern of grooves and ridges in the PTFE surface, whose surface was in turn fibrillated. This combination of features has been found to be particularly beneficial in coating and wiping rolls in a fuser system. The porous polytetrafluoroethylene structure of the present invention provides excellent wear characteristics in comparison to conventional materials and does not shed fibres. In the case of a continuous web, the porous polytetrafluoroethylene structure can be provided much thinner than conventional felts for a given oil-holding capacity, which allows much longer lengths to be used, whilst maintaining the same oiling and wiping characteristics. There is also reduced frictional drag on the rollers. The porous polytetrafluoroethylene structure may be produced as described in patent specification GB 2242431. It is particularly preferred to form the structure from a mixture of particles of different grades of granular- type polytetrafluoroethylene. As is well known, PTFE is produced in two distinct types so-called "granular PTFE" and so-called "fine powder PTFE". These materials have quite different properties and the present invention is concerned with the former. A particularly useful product for use in the present invention comprises 40 to 60% of Teflon (Trademark) resin grade 7A; and 40 to 60% of Teflon resin grade 9B. For a continuous web the content of grade 7A may be increased up to 100%. Teflon resin grades 7A and 9B are available from Du Pont Speciality Polymers Division, Wilmington, U.S.A. The porous polytetrafluoroethylene structure is usually prepared by spraying onto a substrate, such as a ceramic tile or sheet of metal. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiments of the present invention will now be described by way of example only. The attached Figures show various fuser roll arrangements as used in photocopying machines, and to which the present invention may be applied. Figure 1 is a schematic elevation of the fuser section of a plain paper copying machine, where release oil is applied by means of an oil-transfer component in the form of an oil-filled continuous web, according to the invention, which also wipes off excess toner; Figure 2 is a schematic elevation of the fuser section of a plain paper copying machine, where release oil is applied to a continuous web according to the invention by an oil supply mechanism; Figure 3 is a schematic elevation of a fuser section, where release oil is applied onto a pressure roll by means of an oil transfer component in the form of a so-called cover wick; Figure 4 is a schematic elevation of a fuser section of a laser printer; Figure 5 is a schematic elevation of a fuser section, where the oil transfer component is in the form of a roller; Figure 6 shows a cross-section through the roller; and Figure 7 is a photomicrograph at 400 x magnification of a section of a fibrillated porous PTFE structure according to the invention. Figures 1 and 2 show the fuser section of photocopying machines which employ a long web to coat and wipe the fuser roll. Figure 1 shows a PTFE-covered fuser roll 2 and a silicone rubber covered pressure roll 4, which are oiled and wiped by an oil-containing oiling web 6 wound on a spool 8, which passes around pressure rollers 10, 12 onto take-up spool 14; Figure 2 shows a similar arrangement, except that oil is applied to the oiling roll or web by means of an oil supply mechanism 16. The web is formed of a PTFE sheet having a fibrillated surface as produced as described in Example 1, which describes a web formed from 50% PTFE grade 9B and 50% PTFE grade 7A. If a thinner web of higher density is required (for example, in order to wind a longer length of web on spool 8 of a given diameter) then the proportion of PTFE grade 7A can be increased, possibly up to 100% grade 7A. Increasing the amount of grade 7A does, however, increase the stiffness of the web. The fibrillated surface of the PTFE sheet is arranged to be in contact with the fuser roll 2. The web is loaded with silicone release oil before being wound onto the spool 8, when it is ready for use. In use the web is advanced slowly from spool 8 onto take-up spool 14. Release oil is delivered from the oil- filled web, a thin coating thereof applied onto the fuser roll as the fuser roll rotates. The web also wipes excess oil and toner from the fuser roll, and this collects on the web. The fibrillated and profiled surface of the web is particularly effective in wiping the fuser roll and preventing undesired spots of toner from being passed onto the paper copies. In both the Figure 1 and 2 arrangements, the PTFE web is preloaded with release oil. However, in the Figure 2 arrangement this is augmented by a further supply of release oil supplied to the web by a conventional oil supply mechanism 16. This enables more release oil to be applied than can be retained in the web. Figures 3 and 4 show the use of the fibrillated surface PTFE sheet produced in Example 1 as a permeation control layer (20 and 44 respectively) to control delivery of oil from a reservoir (26 and 40 respectively) loaded with release oil. Figure 3 shows a PTFE covered fuser roll 2 and silicone rubber covered pressure roll 4 to which silicone oil is applied by oil supply mechanism 16, reservoir wick 26, and a so-called cover wick 20. The cover wick is supported on a pair of rods 22, 24. In the Figure 3 embodiment, the reservoir is formed of a felt or open-cell foam material (e.g. a melamine foam as disclosed in our W093/20483) and is generally preloaded with release oil. Additional oil is supplied to the reservoir by means of a conventional oil supply mechanism 16. In the arrangement shown, the cover wick 20 is formed of a PTFE sheet having a fibrillated surface as produced in Example 1. The fibrillated surface is arranged to be in contact with the silicone covered roll 4. In conventional manner the cover wick 20 includes a pair of seams (not shown) into which are fitted a pair of rods 22,24 which support the wick in the photocopying machine. In use, release oil is delivered from the reservoir wick 26 and passes through the porous PTFE cover wick in a metered amount onto the silicone covered roll 4. Release oil transfers from the roll 4 onto the fuser roll 2 as the rolls rotate in contact with each other. Excess toner which collects on the roll 4 is wiped off by the cover wick 20, which is replaced after a specified period of use (or number of copies made) . Figure 4 shows a laser printer arrangement. An oil- containing reservoir 40 formed of felt located in channel 42 applies oil to a PTFE-covered fuser roll 2. The reservoir of felt (or open cell foam) has a sheet 44 of porous PTFE produced in Example 1 laminated to the surface thereof by means of a pattern of dots of a heat- resistant adhesive (e.g. Dow Corning RTV 732) . The PTFE sheet 44 is adhered to the top surface of the reservoir and extends partially down the sides thereof. The fibrillated surface of the PTFE sheet is arranged to contact the fuser roll 2 in use. In this case, the reservoir is filled with a supply of release oil, and the reservoir and PTFE sheet assembly is discarded after a predetermined period of use (or number of copies made) . The fibrillated surface acts to supply release oil to the fuser roll and to wipe off excess toner, as above. Figure 5 shows the use of a roller 30 to coat the PTFE-covered fuser roll 2 with release oil and to wipe off excess toner. The roller may be an oil-filled type comprising a reservoir of a felt or open-cell foam mounted on a rotatable spindle, and having an outer oil- permeation control layer of porous PTFE sheet produced as in Example 1. The fibrillated surface of the PTFE sheet is arranged to contact the fuser roll. Alternatively, the roller 30 may be arranged to contact the silicone rubber covered pressure roller 4. The roller may be pre-oiled, or oil may be supplied into the centre of the roller by an oil supply mechanism. Figure 6 is a cross-section through an oil-fed roller 30 and shows the construction thereof. The roller has a hollow metal core 50 defining an inner void 52 into which silicone release oil is supplied in use. The core is provided with a network of apertures (not shown) therethrough for delivery of silicone oil. A sleeve 54 of an open cell melamine foam (see our published patent application PCT/GB93/00563) is fitted around the core. The sleeve is milled from a solid block of open-cell polyurethane and slid over the core. An outer layer 56 of porous PTFE sheet material produced according to Example 1 is wound around the melamine foam sleeve and adhered thereto by means of a heat resistant silicone resin (Dow Corning RTV 732) . The resin is applied along the axially-opposed margins of the layer 56; and along the overlapping axially-extending edges to form a seam 60. The resin cures at room temperature. The PTFE sheet is applied with the fibrillated surface outermost. The roller may also be used as an oil-filled roller by prefilling the inner void 52 with release oil through one of its ends and sealing the end. Figure 7 is a photomicrograph at 400x magnification of a section through a fibrillated PTFE structure according to the invention. It is typical of a structure produced according to Example 1 by deposition of PTFE onto a stainless steel Panzer weave mesh such as size (i) given therein. The photomicrograph shows a view along the direction of the thicker wires of the Panzer weave, and the residual outline 70 left by two of the thicker wires can be discerned. In the Panzer weave the thicker wires form a parallel array whilst the thinner wires run over and under the thicker wires, one thinner wire running over a given thick wire and the adjacent thinner wire running under. Where the PTFE resin has penetrated around a wire (particularly a thinner wire) prior to baking and curing, removal of the PTFE structure from the mesh results in highly fibrillated portions 72 regular surface features i.e. protrusions 74 as a result of tearing of the PTFE as it is removed. The fibrillated portions may include flat web-like features which assist toner wiping. As will be shown later in Example 2, if such penetration is prevented, a less-fibrillated PTFE structure of much inferior toner wiping characteristics is obtained. The overall thickness of the PTFE sheet is about 750 microns, of which almost half (about 350 microns) is generally a fibrillated portion. Viewed in a direction at right angles to that shown in Figure 7 (i.e. along the line of the thinner wires) the protrusions 74 are of generally triangular shape corresponding to triangular voids created where each thinner wire meets each thicker wire, and form a regular surface pattern of features. EXAMPLE 1. (production of PTFE sheet) (a) Preparation of unsintered granular PTFE suspension. 250 grams of DuPont (trademark) granular PTFE grade 9B which had been milled to a weight average particle size of about 40 microns (particle size distribution 99.5% less than 124 microns, and zero% less than 2.8 microns), 250 grams of DuPont granular PTFE grade 7A, 13 ml of Zonyl (trademark) FSN 100 surfactant (a non-ionic perfluoroalkyl ethoxylate mixture) , 10ml of isopropyl alcohol, 41 ml of Pluronic (trademark) L121 surfactant (a polyoxyethylene/polyoxypropylene block copolymer) , and 1.14g of a sodium carboxy- ethyl-cellulose dissolved in water (thickening agent) as a 1% wt/wt solution; were added to 460 grams of water and blended in a Waring blender for 5 minutes. (b) Formation of porous granular PTFE structure.The above suspension was sprayed onto a sheet of woven stainless steel mesh tack-welded onto a stainless steel plate using a Binks BBR spray gun and L88 pressure cup, then dried in an oven. The PTFE suspension generally engulfed the thinner wires of the stainless steel mesh, without surrounding the thicker wires. The air temperature was brought up to 115°C and when the temperature of the stainless steel plate had reached 100°C drying was continued for 30 ins. The air temperature was then progressively increased over a few hours up to 365°C. Once the plate temperature had reached 350°C drying was continued for a further 2.5 hrs. The resultant structure was then cooled. The specific gravity was 0.80 g/cc. Stainless steel meshes of the following sizes were used: (i) 132 wires per inch (0.2 mm dia)/32 wires per inch (0.45 mm dia) Panzer weave (80 micron nominal aperture size; obtained from United Wire, Edinburgh, UK) . (ii) 110 wires per inch (0.23 mm dia)/24 wires per inch (1.06 mm dia) (from Bedfords, Steer, End and Co. Ltd., St. Helens, UK), (iii) 250 wires per inch (0.1 mm dia)/50 per inch (0.51 mm dia) (from Bedford, Steer, End and Co. Ltd.) . The meshes (ii) and (iii) were of Hollander weave. After the structure had cooled, the PTFE sheet attached to the mesh was carefully peeled from the stainless steel mesh. The PTFE had become anchored into the apertures in the mesh, and the process of peeling off the PTFE sheet caused the surface of the PTFE to become torn and fibrillated. Inspection of the surface showed irregular fibrils on the torn surface, which were generally of a length of up to 0.3-0.5mm depending on the particular mesh employed. The nominal thickness was 750 microns. In use, the larger ridges in the PTFE surface (corresponding to the fewer wider wires in the mesh) were arranged perpendicular to the direction of movement of the PTFE sheet through the fuser roll system (i.e. perpendicular to the direction of movement of a web, or parallel to the axis of a roll) . The PTFE sheet may be used in the form of a continuous web 6 (see Figures 1 and 2) , in which case the sheet is wound onto a spool 8 with the fibrillated surface outermost for contact with the fuser roll 2. The larger ridges in the PTFE sheet are arranged to extend transversely of the length of the web so as to assist collection of excess toner wiped from the fuser roll 2 in use. The PTFE sheet may be formed from varying proportions of PTFE grade 7A and 9B as disclosed in GB2242431. The PTFE sheet may also be used as an oil control layer on the outside of a roll 30 or reservoir 26 or 40. A roller construction according to the invention is shown in Figure 6. T fAM f * . * F: ? (comparative performancei In this Example, a PTFE structure of the invention was compared to a comparative PTFE structure without fibrillated portions, in respect of its performance in a photocopying machine. A PTFE sheet of the invention was produced generally in the manner described in Example 1 using a stainless steel panzer weave mesh. In this case the mesh was held in a frame (rather than being tack-welded onto a plate) . The baking time was in the range 2.5 to 6hrs, and the overall thickness of the fibrillated PTFE sheet was 600- 700 microns. A comparative PTFE sheet without fibrillated portions 72 was produced in analogous manner as follows. In order to prevent penetration of the PTFE suspension into the mesh, the mesh was first sprayed with a solution of carboxymethyl cellulose, and the coating allowed to dry. The coating was sprayed onto the mesh from above in such manner that the carboxymethyl cellulose ran through the mesh and coated the lower surface thereof. The mesh was then sprayed from above with a PTFE suspension as described in Example 1. The carboxymethyl σβllulose coating formed a barrier to penetration of the subsequent PTFE suspension right through the mesh; and in particular the coating prevented the PTFE suspension in large degree from engulfing the thinner wires of the mesh. This substantially reduced the amount of fibrillation in the finished sheet. The PTFE was then dried and baked as before and the resultant sheet peeled from the mesh. To assess performance, continuous webs of the PTFE sheets were tested in a Siemens ND2 printer (100 copies/min) under the same standard operating conditions. The arrangement is analogous to that shown in Figure 2, except that the direction of rotation of the fuser roll was anti-clockwise and a doctor blade 80 (shown in dashed lines) was provided between the web and the nip of the rollers to collect any excess toner not collected by the web before it was carried into the roller nip area. The PTFE web was advanced at a mean rate of about 0.2mm/min. The oil feed rate via oil supply mechanism 16 was constant. It was found that the fibrillated PTFE web of the present invention efficiently oiled the fuser roll 2 and also removed excess toner from the roll in satisfactory manner. However, the comparative non-fibrillated PTFE web did not satisfactorily remove excess toner from the fuser roll, with the result that excess toner built up on the doctor blade, to the extent that the trial was stopped prematurely. Thus, the fibrillated PTFE structure of the present invention showed superior toner wiping properties in comparison to the comparative sheet. Testing and Preparative Methodologies 1) The specific gravity of the PTFE sheet is determined by weighing a sample thereof in two different media, viz; air and water. The weights were determined using an Avery VA124 analytical balance. The specific gravity is calculated as being equal to (weight in air)/(weight in air - weight in water) . 2) TEFLON (trademark) granular-type PTFE fluorocarbon resin grades 7A and 9B are available from DuPont Speciality Polymers Division, Wilmington U.S.A. Grade 9B is a premelted sintered resin. The manufacturers product specification indicates an average specific gravity of 2.16, and an average particle size of 35 microns (grade 7A) and 500 microns (grade 9B prior to milling) . PTFE grade 7A was unsintered and was used as supplied. Prior to use, the PTFE grade 9B was milled to a weight average particle size of about 40 microns by grinding an aqueous slurry thereof between grinding stones at room temperature as follows. The PTFE grade 9B was mixed with water to form a slurry, and the slurry fed between closely spaced grinding surfaces of a grinding mill as disclosed in US- A-4841623, to crush and shear the pieces of PTFE into particles. The ground slurry was then filtered or centrifuged to separate the porous expanded PTFE particles from water, and the separated finely ground particles were oven dried at from 125°C - 150°C. 3) Particle size of ground PTFE grade 9B was determined as follows: using a magnetic stirrer and ultrasonic agitation, 2.5 grams of milled PTFE powder were dispersed in 60 ml isopropyl alcohol. (Ultrasonic Probe Model W- 385, manufactured by Heat Systems-Ultrasonics, Inc.). Aliquots of 4-6ml of the dispersed particles were added to approximately 250ml of circulating isopropyl alcohol in a Leeds & Northup Microtrac FRA Particle Size Analyzer of analysis. Each analysis consisted of three 30-second runs at a sample circulation rate of 2 litres/minute during which light scattering by the dispersed particles is automatically measured and the particle size distribution automatically calculated from the measurements.

Claims: CLAIMS 1. A porous polytetrafluoroethylene (PTFE) structure formed from particles of granular-type polytetrafluoroethylene fused together such as to form a porous integral network of interconnected particles, the structure having a fibrillated (72) surface. 2. A structure according to either preceding claim wherein the fibrils (72) have a length up to 1mm. 3. A structure according to any preceding claim wherein the fibrils have a length up to 0.5mm. 4. A structure according to any preceding claim wherein the fibrillated surface has additionally an overall pattern of regular surface features (74) . 5. A structure according to claim 4 wherein the surface features are selected from ridges, grooves, protrusions and indentations. 6. A structure according to claim 5 wherein the protrusions (74) are generally triangular in shape. 7. A structure according to claim 5 or 6 wherein the fibrillated surface bears the imprint of a wire mesh. 8. An oil transfer component for coating or wiping a roll (2) in a fuser system of a copying machine, which comprises; a porous polytetrafluoroethylene (PTFE) structure formed from particles of granular-type polytetrafluoroethylene fused together such as to form a porous integral network of interconnected particles, the structure having a surface for contacting said fuser system roll in use, the surface being fibrillated (72) . 9. A component according to claim 8 wherein the porous PTFE has a specific gravity of 0.5 to 1.8. 10. A component according to claim 9 wherein the porous PTFE has a specific gravity of 0.7 to 1.2. 11. A component according to any of claims 8 to 10 wherein the porous PTFE is free of filler materials of mineral origin. 12. A component according to any of claims 8 to 11 wherein the PTFE component has been produced by spray deposition. 13. A component according to any of claims 8 to 12 wherein the porous PTFE structure is in the form of a sheet material. 14. A component according to claim 13 wherein the PTFE sheet material is laminated to a woven or non-woven backing fabric. 15. A component according to claim 13 or 14 wherein the PTFE sheet material has a thickness of 50 to 1500 microns. 16. A component according to any of claims 8 to 15 which is pre-loaded with oil. 17. A component according to claim 16 wherein the release oil comprises 10 to 50% by weight of the component. 18. A component according to any of claims 8 to 17 in the form of a roll (6) of sheet material. 19. A component according to any of claims 8 to 17 in the form of a cover wick (20) . 20. A component according to any of claims 8 to 17 in the form of a roller (30) having an outer layer of said porous PTFE material. 21. A component according to any of claims 8 to 17 which comprises an oil reservoir material (40) having laminated thereto the porous PTFE material (44) . 22. A method of coating or wiping a roll in a fuser system of a copying machine which comprises; providing a porous polytetrafluoroethylene (PTFE) structure formed from particles of granular-type polytetrafluoroethylene fused together such as to form a porous integral network of interconnected particles, the structure having a fibrillated surface; arranging the fibrillated surface of the porous PTFE structure to be in contact with the roll in said copier fuser system; and operating the copying machine such that the porous PTFE structure is effective in coating the roll with oil and/or wiping off excess oil or toner from the roll. 23. A method of manufacturing a porous polytetrafluoroethylene (PTFE) structure having a fibrillated surface, which comprises; applying particles of granular-type polytetrafluoroethylene to a substrate having surface features; fusing the particles to form a structure in the form of a porous integral network of interconnected particles mechanically bonded to the substrate surface features; and removing the structure from the substrate and thereby forming a fibrillated surface in the PTFE structure. 24. A method according to claim 23 wherein the substrate is a woven mesh. 25. A method according to claim 24 wherein the mesh has a weave comprising thicker wires and thinner wires. 26. A method according to claim 24 wherein the fused PTFE structure bonded to the substrate mesh substantially engulfs the thinner wires thereof.

Inventors: ALLAN ANDREW MILLER, MCCOLLAM FRANCIS MICHAEL JOHN, ALLAN, ANDREW, MILLER, MCCOLLAM, FRANCIS, MICHAEL, JOHN
Applicants: ALLAN ANDREW MILLER, GORE W L & ASS UK, MCCOLLAM FRANCIS MICHAEL JOHN, ALLAN, ANDREW, MILLER, MCCOLLAM, FRANCIS, MICHAEL, JOHN, W.L. GORE & ASSOCIATES (UK) LTD.
Application year: 1994
Main classification: B29C41
IPC codes: B29C33, B29C41, B29C59, B29C67, C08J9, G03G15